Field of the Invention
The invention relates to a safety system for an inductive power transfer system, in particular an inductive power transfer system for transferring electric energy to a vehicle which is standing or travelling on a surface of a route. Furthermore, the invention relates to a method of operating such a safety system and a method of building such a safety system.
Description of Related Art
WO 2012/047779 A1 discloses a safety system for a charger to provide protection with respect to an object that may become hot during operation of the charger, wherein the safety system comprises a detection subsystem configured to detect presence of the object and substantial proximity to the charger and a notification subsystem operatively coupled to the detection subsystem and configured to provide an indication of the object. The publication discloses that one or more inductive sensors can be integrated into a source device, source housing, vehicle, or surrounding area to detect obstructions and foreign objects and/or materials between the source and device resonators.
WO 2009/081115 A1 discloses a primary unit for use in an inductive power transfer system, the primary unit being operable to transmit power wirelessly by electromagnetic induction to at least one secondary unit of the system located in proximity to the primary unit and/or to a foreign object located in set proximity, wherein the primary unit comprises driving means operable to drive the primary unit so that in a driven state the magnitude of an electrical drive signal supplied to one or more primary coils of the primary unit changes from a first value to a second value. Furthermore, the primary unit comprises means for assessing the effect of such driving on an electrical characteristic of the primary unit and means for detecting in dependence upon the assessed effect the presence of said secondary unit and/or foreign object located in proximity to said primary unit.
EP 2317625 A2 discloses a primary device for inductive power transfer to a secondary device, wherein the primary device comprises a primary coil, wherein the primary device is being configured to (i) operate in a first mode during which the primary coil transfers power to inductive coupling to the secondary device and (ii) operate in a second mode during which a foreign object is detected. Furthermore, a primary control is configured to operate the primary coil (i) using a first frequency during the first mode and (ii) using a second frequency during the second mode.
DE 20 2009 009 693 U1 and the DE 10 2009 033 236 A1 disclose a device for inductive transmission of electrical power from a stationary unit with at least one primary inductance to a vehicle adjacent to this stationary unit having at least one secondary inductor, wherein the stationary unit comprises means for detecting the presence of an electrically conductive object within a predetermined space adjacent to the primary inductor.
It is an object of the present invention to provide the safety system for an inductive power transfer system, a method of operating such a safety system and a method of building a safety system which provides reliable and fast detection of a foreign object, in particular a metal object, located in proximity of a primary winding structure of the primary winding.
It is a basic idea of the present invention that a foreign object located in the proximity of a primary winding structure of the primary unit will cause a change in electrical characteristics of a detection structure, in particular a change of an inductance of the detection structure.
The present invention can be applied to the field of energy transfer to any land vehicle, including track bound vehicles, such as rail vehicles (e.g. trams), but also to road automobiles, such as individual (private) passenger cars or public transport vehicles (e.g. busses). A problem in such devices is that it is generally not possible to mechanically prevent foreign objects, in particular objects made of metal, from being placed into proximity of the primary unit of an inductive power transfer system. Such foreign objects may e.g. comprise a coin, a can, a key, a tool and other objects. The varying magnetic field generated by the primary unit and a secondary unit may induce current in the foreign objects made of metal and in other objects or fluids. Such currents may cause power losses and heating of the object. Heating of the foreign objects made be dangerous for example to persons trying to touch and remove the foreign object and/or may damage the surface the foreign object is placed on or parts of the primary unit. Also, a heated object can cause fire.